The present invention relates to a method of automatic gear selection in a vehicle power transmission, the gear selection being carried out by a control system including an electronic unit which, in response to signals representing vehicle speed and engine load, coacts with a read-only memory (preferably a PROM) in which there are stored, for each gear position, alternative sets of speed limits for changing gear to neighboring gear. Which set of speed limits will be used to cause the gear to change is determined by limiting values relating to speed change (acceleration or deceleration) and engine load.
Arrangements for automatic gear selection are already known, but such systems are primarily intended for automatic gearboxes with a hydraulic torque converter. In automatic gear selection systems for purely mechanical transmissions, where gear changing is first enabled after the main clutch has interrupted the torque connection in the engine, problems often arise in obtaining a correct gear selection, e.g. when changing gear on a hill. Particularly with heavy vehicles, it is necessary that control of the engine load be accurately synchronized with the main clutch at the beginning of a gear changing process. In the absence of such synchronization, the vehicle may be subjected to an appreciable change in speed before the torque connection is completely interrupted. Such a change in speed could cause signals fed into the gear selection system to bring out the selection of an unnecessarily low gear, which can be troublesome for the driver, and furthermore affect traffic rhythm negatively.
The present invention has the task of providing a gear selection system where the risk of unsuitable gear selection has been eliminated. The invention is distinguished in that during travel, data on speed change (acceleration or deceleration--hereinafter referred to simply as acceleration) and engine load are stored in a random access memory (RAM); that the speed value most recently stored in said RAM before the initiation of the gear selection is compared, together with values stored in said RAM and relating to acceleration and engine load at a predetermined time period prior to the initiation of the speed change, with values stored in said PROM and relating to speed, acceleration and engine load, respectively; that said comparison results in the choice of a certain gear; and that in the electronic unit there is generated an output signal representing the selected gear.
In the inventive method, the input signals generated in the gear selection system immediately prior to the initiation of the gear selection process are not used for selecting a gear. There are thus avoided undesirable variations in the input signals, which could possibly occur immediately prior to the initiation of the gear change process.
In an advantageous embodiment of the invention, values for acceleration and engine load relating to an instant at least 0.5 seconds before the initiation of gear selection are used in the gear selection process.